A converter of the kind mention above is illustrated, for example, in German Pat. No. DE 2,644,715 in FIG. 25. Large power losses occur in such a converter, particularly when converting high voltages and at high power take-off. The losses occur particularly in the diode, through which a reverse current occurs each time the switching element closes. Losses in the double also occur when it is in on-state condition in the forward direction. These so-called on-state losses dominate at comparatively low switching frequencies for example at the supply frequency (50-60 Hz). On the other hand, the switching-off losses of the diode dominate when hacking at high frequency, e.g. 20 kHz, particularly in conjunction with high voltages, e.g. 400 volts, and high current strengths. Apart from the disadvantages which the losses intrinsically are, they also result, together with the heavy voltage and current variations occurring in the converter, in that the diode and switch are subjected to large stresses and are easily damaged. The current variations also give rise to radio interference, which can disturb the apparatus in which the converter is included and other apparatus as well.
The known converter according to the above has been provided with a protective circuit for decreasing the power losses and high voltages at the switch. However, it has been found difficult to get a circuit of this kind to function satisfactorily. Furthermore, the losses are increased due to energy that has been preciously stored in coils capacitors, being subsequently consumed in resistors.